The invention relates to a display tube comprising a display window having two short sides and two long sides, and a color selection electrode having a longitudinal axis and a transverse axis and supporting means for supporting the color selection electrode in tension, which color selection electrode comprises a central portion having apertures allowing passage of electrons, and a circumferential part, which circumferential part is provided with vibration damping means.
A color display tube comprising a color selection electrode customarily includes a display screen having a pattern of lines or dots of an electroluminescent material, and an electron gun for generating electron beams, the color selection electrode, which may comprise a mask of the wire-mask or shadow-mask type, being arranged between the electron gun and the display screen. The mask is formed by, for example, a thin metal foil having a large number of apertures, which must be very accurately arranged so as to be very close to the display screen, so that the mask apertures are systematically aligned with respect to the luminescent lines or dots of the display screen. To maintain this systematic alignment, the mask must be suitably suspended in the color display tube, in order to make sure that the mask remains in an accurately determined position during the manufacture of the tube as well as during operation of the tube. To achieve this, a frame is used as a supporting device, which frame carries the mask and is suspended in the tube. The frame is generally rectangular comprising two short sides and two long sides, a transverse axis parallel to the short side, and a longitudinal axis parallel to the long side.
In the case of a mask with a single curvature (for example a cylindrical curvature), a frequently used construction comprises a rigid, heavy frame provided with the proper curvature, after which the mask is clamped to the curved sides. Also in the case of a flat mask, an example of a frequently used construction is that wherein the tension mask is supported by a frame.
Examples of such tension masks are the masks used in the Sony Trinitron(trademark) tube and the Zenith FTM(trademark) (flat tension mask) tube. The FTM tube employs a so-called dot screen, wherein the phosphor elements are provided in the form of triads of red, blue and green dots, which requires aligning by, and application of, mechanical tension in both the longitudinal and the transverse direction of the mask.
The Sony tube employs a so-called striped screen, wherein the phosphor elements are arranged in the form of juxtaposed triads of red, blue and green stripes, as a result of which aligning is necessary only in the direction transverse to the stripes. The Sony mask is a grid structure of grid elements clamped to a rectangular frame so as to extend parallel to the transverse axis. The grid elements are clamped between the supports of the frame in such a manner that even heating and expansion does not cause them to become slack.
Display tubes currently being developed, in particular (flat) display tubes of the striped-screen type employ so-called slotted masks, wherein a number of comparatively long slots are situated below each other in the longitudinal direction.
In all of the above cases, the mask is a very thin, not self-supporting foil, which is supported by the frame while being subjected to a fairly high tension.
Both the cylindrical and the flat masks of this type can easily be set vibrating, which may be caused, for example, by external shocks or by a loudspeaker situated in the vicinity of the display tube. The resonant frequency of the mask vibrations depends on the mechanical parameters and the tension in the mask. Each vibration of the mask will result in electron beams not impinging on the associated phosphor elements, which leads to color impurities in the image displayed.
Various means for damping said vibrations have been proposed.
In the case of cylindrically curved tension masks, for example, a damping wire is clamped across the grid elements of the mask in a direction parallel to the longitudinal axis. In such an arrangement, the damping wire exerts a spring pressure on the grid elements, so that they are less easily set vibrating by external, mechanical shocks. This solution can be applied to (cylindrically) curved masks, but not to flat tension masks.
In Patent Abstracts of Japan, publication number 11054061, a (flat) mask is described which is clamped in two directions. The vibration damping means comprise a number of movable, cylindrical parts arranged along the four circumferential sides and fixed parts of wire material which extend through the movable parts and are secured to the mask so as to be under tension in order to press the movable parts against the mask. This arrangement is voluminous, its manufacture is rather complicated, and adjusting it so as to suppress a specific, disturbing vibration frequency is difficult.
It is an object of the invention to provide a damping-means arrangement which can suitably be applied to both flat and curved masks, which takes up little space, is easy to make and, preferably, also easy to adjust.
To achieve this, a display tube of the type described in the opening paragraph is characterized in that the vibration damping means include a resonator in the form of at least one metal tongue, which is capable of vibrating when the color selection electrode vibrates, and is arranged in such a way, and/or co-operates with such means, that it is capable of exchanging vibration energy by means of collision and/or friction with the circumferential part of the color selection electrode.
The use of one or more tongues as vibration damping means, which, for example, can be formed from the mask itself (for example by etching) or which can be welded onto the mask has various advantages.
These tongues are easy to make (in particular if they are etched from the mask) and can be readily provided. By suitably choosing their dimensions (in particular the length dimension) and their mass, they can be adjusted to a desired resonant frequency. If the tongues extend in, or parallel to, the plane of the mask, they take up little space. By making them collide with or rub against the mask, they can exchange vibration energy without additional parts being required, as in the arrangement described in, for example, U.S. Pat. No. 4,827,179, which arrangement includes flexible lugs, which extend transversely to the plane of the mask and are welded onto a bracket, and xe2x80x9cfriction brakesxe2x80x9d connected with said lugs. Without such xe2x80x9cfriction brakesxe2x80x9d, the vibration is not effectively damped.
It will be clear that the arrangement in accordance with the invention can be applied to curved as well as flat tension masks. To function properly, it is important that the color selection electrode comprises parts which are mechanically connected to one another and extend parallel to the transverse axis. Particularly dot masks and slotted masks meet this requirement.
Preferably, one or more tongues are provided on the mask near one, or both, short sides of the display window because at said location the operation of said tongue(s) is most effective.
In a simple embodiment which takes up little space, the tongue, or tongues, are formed from the material of the circumferential part of the mask (for example by means of etching) and have four sides, three of which are separated from the material of the circumferential part by slots. Such a tongue is situated in, or extends parallel to, the plane of the mask and is capable of vibrating in the opening formed during the formation of the tongue. By providing stopping means at one side of the tongue, i.e. at the tongue itself, or at the mask, the tongue can be made to collide with the mask when it moves in the direction of said mask.
In a practical embodiment, the tongue comprises a free, folded end portion. By virtue thereof, it is possible, on the one hand, to adjust the resonant frequency of the tongue. On the other hand, the folded portion can readily be used as the stop if it is wider than the opening wherein the non-folded portion extends.
Instead of folding a free end portion, the same object (providing additional mass or forming a stop) can also be achieved by providing the tongue with a welded-on plate.
The stop enables the tongue to move back and forth on one side of the mask. To optimize vibration damping, use is advantageously made of two tongues, the first one of which is arranged such that, during a vibration, it can move back and forth only on one side of the color selection means, the second tongue being arranged such that, during a vibration, it can move back and forth only on the other side of the color selection electrode,
An alternative to forming one or more tongues by etching is the use of one or more separate tongues which are welded onto the mask with one end portion. This has the advantage that, as a result of the material choice and the dimensions, the spring characteristics of the tongue can be chosen more freely. A tongue having a wider free end portion than the welded end portion may offer advantages, for example, in that respect. Also in this case, use is effectively made of at least two tongues, each forming a separate part, one tongue being welded with one end portion on one side of the color selection electrode and the other tongue being welded on the opposite side of the color selection electrode.
In the case of sinusoidal movements of the mask, a deflection to one side can be damped by one half of the tongues, while a deflection to the other side can be damped by the other half of the tongues.
The above-described embodiments are based on the principle of energy exchange through collision. Although this seems to be the most effective way, it is alternatively possible, within the scope of the invention, to exchange energy through friction. For this purpose, an embodiment of the display tube in accordance with the invention is characterized in that a first part of the tongue is bent so as to extend outside the plane of the color selection electrode, and a second part is bent back again towards said plane and is capable of moving through an opening in said tongue, its movement being limited by the friction against one or more walls of the opening.
The invention also relates to an advantageous method of manufacturing a color selection electrode for a display tube, characterized in that a metal foil is provided and in that openings allowing passage of electrons are etched in a central portion of the foil and, simultaneously, at least one U-shaped slot is etched in a circumferential part of the foil, the material inside the slot forming the tongue.
By providing, in one etch process, both the openings allowing passage of electrons and slots defining a tongue or tongues, the provision of the vibration damping means in accordance with the invention hardly requires additional costs or additional effort.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.